Image signal sensing process including altering charges of capacitances

ABSTRACT

In a method for detecting an image signal by means of an array ( 30 ) of photosensitive devices ( 10 ) with each of which the charge of a capacitance ( 12 ) can be altered, an optical signal ( 20 ) of each photosensitive device is first detected by means of the following substeps: creating a charge condition of the capacitance with a predetermined voltage (+U_DD), changing the charge of the capacitance ( 12 ) either with a photocurrent generated in the photosensitive device ( 10 ) by the optical signal ( 20 ) or with a quantity derived from the same and detecting the voltage across the capacitance ( 12 ) after a predetermined time period, deciding whether the detected voltage lies within a valid range and, if this is so, determining on the basis of the detected voltage a valid signal which characterizes the detected optical signal, and, if this is not so, repeating the cited steps with a time period which differs from the predetermined time period either a predetermined number of times or until it is determined that the detected voltage lies in a valid range. Subsequently the optical signal detected for each photosensitive device ( 10 ) is stored together with the time period for which a valid signal has been detected. Finally the image signal is obtained from the stored optical signals for the individual photosensitive devices and the respective assigned time periods.

DESCRIPTION

The present invention relates to a method for detecting an image signalby means of an array of photosensitive devices with each of which thecharge of a capacitance can be altered.

In monolithic integrated image sensor systems, arrays of sensor elementsare usually fabricated together with the electronics for signal readouton an integrated circuit (IC). In the field of integrated opticalsensors, photodiodes, bipolar phototransistors, charge-coupled devices(CCDS) and light-sensitive MOS transistors can be used aslight-sensitive devices. The light-dependent signals, i.e. the charge,the voltage or the current, of the arrays of the above-named devices arenormally read out and subsequently converted to a digital signal andprocessed further. Alternatively, these light-dependent signals can bedisplayed directly without an analogue/digital conversion.

The process of reading out image signals by means of an integratingmethod is known. For example, a signal current of a light-sensitivedevice is here integrated onto a capacitance over a certain period oftime. As a result of this integration, a signal voltage proportional tothe illumination intensity and to the time period is generated, whichcan then be read out easily from a peripheral circuit on the integratedcircuit. In one example of a light-sensitive device according to theprior art, a capacitance which was initially charged with apredetermined potential is discharged by a photocurrent generated by thelight-sensitive device.

In some applications, for example in the automobile branch, where thereare frequent changes in brightness conditions and strong differences inbrightness within an image, the demands made on the dynamics of theimage signals are very great. Image signal dynamics of this order, aconsequence of too strong a variation in the illumination intensitywithin an image, cannot be achieved by the method according to the priorart described above. If, for example, the illumination intensity is toohigh for certain image sensor elements, the voltage of the capacitanceinitially charged with a predetermined voltage drops very quickly, sothat, after the time period during which the capacitance is dischargedby the signal current of the light-sensitive device has elapsed, it isno longer possible to assess how high the incident illuminationintensity was initially if the capacitance has discharged completely ontermination of the time period. The known method thus exhibits a limiteddynamic range.

U.S. Pat. No. 4,479,062 describes an apparatus for photoelectricalconversion in which an array of light-receiving elements is provided inorder to accumulate information relating to incident light. The knownapparatus has a saturation detection device for detecting a saturationof an output signal of the light-receiving element array. If the outputsignal of the light-receiving element array is saturated, theaccumulation time is reduced. According to U.S. Pat. No. 4,479,062 thedetection time is reduced progressively until none of theimage-receiving elements of the image-receiving element array isoversaturated, i.e. until the output signal of the light-receivingelement array no longer exceeds a saturation level. When the outputsignal of the light-receiving element array no longer exceeds thesaturation level, all the image sensor elements provide a valid signal,whereupon the image acquired with the integration time which wasascertained last is then evaluated.

The method known from U.S. Pat. No. 4,479,062 is disadvantageous inthat, despite the effort involved in multiple image acquisition, theresulting image may have image sensor elements having a low signal/noiseratio, e.g. image sensor elements with low brightness values. For anobserver, however, it is advantageous if the signal/noise ratio for anacquired real scene is as large as possible. For the observer a largesignal/noise ratio means that objects which are static and not subjectto fluctuations in illumination really do have a static appearance inthe image, and that it is possible to resolve low contrasts within ascene. A further disadvantage of the known method is that the dynamicrange for the totality of all the image sensor elements of the acquiredimage is restricted to the physically limited dynamic range, i.e. theratio of the maximum signal to the equivalent noise signal, of a singleimage sensor element for an image acquisition. For an observer, a highdynamic range means that both very bright and also very dark regions ofa real scene can be represented without appearing equally bright orequally dark from a certain threshold on.

It is the object of the present invention to provide a method fordetecting an optical signal that exhibits an increased dynamic range andincreased precision.

This object is achieved by a method according to claim 1.

The present invention provides a method for detecting an image signal bymeans of an array of photosensitive devices, with each of which thecharge of a capacitance can be altered, wherein at first an opticalsignal of each photosensitive device is detected by means of thefollowing substeps: creating a charge condition of the capacitance witha predetermined voltage, changing the charge of the capacitance eitherwith a photocurrent generated in the photosensitive device by theoptical signal or with a quantity derived from the same and detectingthe voltage across the capacitance after a predetermined time period,deciding whether the detected voltage lies within a valid range and, ifthis is so, determining on the basis of the detected voltage a validsignal which characterizes the detected optical signal, and, if this isnot so, repeating the cited steps with a time period which differs fromthe predetermined time period either a predetermined number of times oruntil it is determined that the detected voltage lies in a valid range.Subsequently the optical signal detected for each photosensitive deviceis stored together with the time period for which a valid signal hasbeen detected. Finally the image signal is obtained from the storedoptical signals for the individual photosensitive devices and therespective assigned time periods.

In an embodiment of the method according to the present invention it isdecided in the decision step that the detected voltage does not lie inthe valid range if the detected voltage corresponds to a completedischarge of the capacitance. In this case the time period after whichthe voltage is detected, which differs from the predetermined timeperiod, is reduced for each repetition.

The method according to the present invention provides an extension ofthe dynamic range in the detection of optical signals, e.g. when imagingby means of an image sensor array consisting of image sensor elements.The method according to the present invention prevents departure fromthe operating range, i.e. an overdriving, of individual image sensorelements and also an overdriving of the complete image sensor array.

According to the present invention it is thus decided, after each imageacquisition, whether a valid signal exists for each image sensorelement. If this is so, the value of the respective image sensorelement, together with the information on the integration time, isstored e.g. in an intermediate memory. This is also carried out even ifthere have already been invalid signals of image sensor elements in thispartial image. The control of the brightness values and information onthe integration time can e.g. be realized by means of a digitalprocessor.

In preferred embodiments of the present invention, acquisition of apartial image commences with the greatest sensitivity, i.e. with thelongest integration time. This ensures that all the image sensorelements exhibit a maximum signal/noise ratio in the prevailingcircumstances. If subsequently, for a curtailed integration time,remaining image sensor elements are still overdriven, i.e. do notexhibit valid signals, the image sensor elements in the intermediatememory can e.g. adopt the maximum representable value or the chosenintegration times can be curtailed, if desired.

The result of this multiple acquistion of images is a composite image ofimage sensor elements which is made up of partial images which may havebeen acquired with different integration times. In order to be able toevaluate the image, it is clear that, in a further step, the brightnessvalues and information on the integration time must be coordinated withone other so that the correct brightness information is obtained foreach image sensor element. This coordination could e.g. be achievedusing software.

The method according to the present invention improves not only thesignal/noise ratio for each image sensor element but also the dynamicrange of the totality of all the image sensor elements contained in theimage. For instance, a single image sensor element may possess a dynamicrange, defined here as the ratio of maximum signal to the equivalentnoise signal, of 60 dB, a typical value for photodiodes. If now severalimages are acquired with different integration times, i.e.sensitivities, the variation of the integration time comprises e.g. 1 msto 10 μs, which corresponds to 40 dB. The resulting dynamic range forthe totality of all the image sensor elements contained in the image,which is obtained by simply adding together both dynamic values, is then60 dB+40 dB=100 dB.

Preferred embodiments of the present invention will be described in moredetail below making reference to the enclosed drawings, in which

FIG. 1 shows an image sensor element which is suitable for use in themethod according to the present invention;

FIGS. 2A and 2B show time schematics of a signal detection to clarifythe method according to the present invention; and

FIG. 3 shows a schematic representation which clarifies the way in whichthe optical signals detected by means of the method according to thepresent invention can be evaluated further.

The present invention will be described below in the light of apreferred embodiment of the same wherein a photodiode is used as imagesensor element. In FIG. 1 a photodiode 10 is shown, the blocking layercapacitance 12 of the same being shown as a discrete element for thesake of clarity. The cathode of the photodiode 10 is connectable to apotential +U_DD via a switch 14. The anode of the photodiode 10 isconnected to a potential of 0V, i.e. ground. The cathode of thephotodiode 10 is further connectable to an output terminal 18 via aswitch 16.

At the start of the readout process, the switch 14 is first closed,while the switch 16 is open, whereby an output voltage U_aus is set backto the voltage U_DD and the blocking layer capacitance 12 of thephotodiode 10 is charged to the voltage U_DD. The switch 14 is thenopened. By means of an illumination 20, which is produced by an opticalsignal, e.g. an image signal with a certain illumination intensity, aphotocurrent is now generated in the photodiode 10 and integrated ontothe blocking layer capacitance 12 of the photodiode 10. Due to thissignal current generated by illumination, the voltage U_aus falls withtime. After a fixed predetermined time period, the switch 16 is closed.The resulting signal voltage U_aus, which is characteristic of theillumination falling on the photodiode 10, can thereby be read out atthe output terminal 18.

The method according to the present invention now consists ofintegrating several times in succession the signal of each image sensorelement in an image sensor array, where the photodiode shown in FIG. 1represents such an image sensor element, and to decide whether each ofthese is a valid signal, different integration times being used for theintegrations each time. A valid signal is available when the outputvoltage U_aus lies within a valid range.

The way in which the voltage U_aus at the output node 18 varies withtime for the image sensor element depicted in FIG. 1 is shown in FIGS.2A and 2B. The image sensor element depicted in FIG. 1 supplies a validoutput signal all the while the blocking layer capacitance 12 has notdischarged itself completely at the moment of readout. In FIG. 2A thephotodiode is subjected to a low illumination intensity, so that, afterthe integration time, i.e. the detection time t_int_1, the blockinglayer capacitance 12 is not fully discharged. Thus, after the timet_int_1, a valid output signal 22 is obtained, which is proportional tothe illumination intensity to which the photodiode is subjected andproportional to the time period over which the photocurrent isintegrated, i.e. during which the capacitance is discharged. A reneweddetection over a shorter time period t_int_2 is not necessary in thiscase, i.e. at the low illumination intensity.

The voltage U_aus against time for a large illumination intensity towhich the photodiode is subjected is shown in FIG. 2B. The capacitanceis thus discharged quickly from the voltage +U_DD to the voltage 0V.Therefore no valid output signal is obtained during the time periodt_int_1. As a result a further detection is performed during a timeperiod t_int_2, which is shorter than the time period t_int_1. At themoment of readout after this second time period the capacitance is alsoalready fully discharged. Thus there is again no valid signal. Only witha further reduction in the detection time period, i.e. t_int_3, is thecapacitance not fully discharged at the moment of detection. Thus avalid output signal is obtained at the end of the third time periodt_int_3.

Through the increasing reduction in the integration time, i.e. in thedetection time period, when the previous detections do not provide avalid output signal, the dynamic range is increased for the totality ofall the image sensor elements in the image.

FIG. 3 shows an apparatus which can be used to evaluate imageinformation acquired by means of an image sensor array. An image sensorarray 30, which consists of individual image sensor elements, each ofwhich detects an optical signal, twenty five of which are shown for thesake of example in FIG. 3, is connected via a control circuit 32 to anintermediate memory 34. In the intermediate memory 34 a particularstorage location can e.g. be allocated to each image element of theimage sensor array 30. The control apparatus 32, as explained above,induces multiple accessing of the image sensor elements of the imagesensor array 30 when necessary. Furthermore, the control apparatus 32decides in each case whether there is a valid signal from the imagesensor elements after the respective detection time. The respectivevalid signals are stored by the control apparatus 32, together with theinformation on the respective integration time, at the respectivestorage location in the intermediate memory 34, i.e. the image memory.For an orderly allocation it is advantageous that each pixel has beenaccessed for each integration time. It is also advantageous to performthis accessing and the writing into the intermediate memory in parallel.The image information, together with the additional information on therespective integration time, can now be read out of the image memory 34and then be processed or displayed.

In an alternative implementation of the method according to the presentinvention to that described above, instead of the long initialintegration time t_int_1 depicted in FIGS. 2A and 2B, a shortintegration time, e.g. t_int_3, can be used at the beginning of themethod. A valid output signal would then be understood as one whosevoltage has fallen by more than a predetermined percentage in respect ofthe voltage +U_DD. This percentage could e.g. be 10% of the voltage+U_DD. If the output voltage lies in the invalid range, i.e. if it hasfallen by less than 10%, the integration time would be increased, e.g.from t_int_3 to t_int_2. In this way the sensitivity of the image sensorelements of an image sensor array could be increased.

It is obvious that other light-sensitive devices could be used as imagesensor elements instead of the photodiode described as long as theoptical signal, e.g. the illumination intensity, is detected via achange in the charge of a capacitance over a predetermined time period.Such alternative devices are e.g. bipolar phototransistors orlight-sensitive MOS transistors.

The variation in the integration times so long as no valid signal isdetected is normally continued until a valid signal is detected.Alternatively, the repetition of the detection with differentintegration times may be performed only a certain number of times,whereupon e.g. a specified value is used for the illumination intensity.

It is obvious that instead of using the photocurrent directly, thecharge of the capacitance can also be changed using a quantity derivedfrom the photocurrent, e.g. one prepared by means of an amplificationcircuit or the like.

Apart from image detection, the present invention can also be used toadvantage in e.g. outdoor supervision methods.

What is claimed is:
 1. A method for detecting an image signal by meansof an array of photosensitive devices with each of which the charge of acapacitance can be altered, comprising the following steps: a) detectingan optical signal of each photosensitive device by means of thefollowing substeps: a1) creating a charge condition of the capacitancewith a predetermined voltage, a2) changing the charge of the capacitanceeither with a photocurrent generated in the photosensitive device by theoptical signal or with a quantity derived from the same and detectingthe voltage across the capacitance after a predetermined time period,a3) deciding whether the detected voltage lies within a valid range and,if this is so, determining on the basis of the detected voltage a validsignal which characterizes the detected optical signal, and, if this isnot so, a4) repeating the steps a1) to a3) with a time period whichdiffers from the predetermined time period either a predetermined numberof times or until it is determined in step a3) that the detected voltagelies in a valid range; b) storing the valid signal, determined for eachphotosensitive device and characterizing the detected optical signal,together with the time period for which a valid signal has beendetected, wherein a predetermined signal is stored if a valid signalcharacterizing the detected optical signal is not detected in steps a1)to a4); and c) obtaining the image signal from the stored opticalsignals for the individual photosensitive devices and the respectiveassigned time periods.
 2. A method according to claim 1, wherein thestep a1) comprises the charging of the capacitance to the predeterminedvoltage and in step a2) the capacitance is discharged by thephotocurrent or by a quantity derived from the same.
 3. A methodaccording to claim 2, wherein in step a3) it is decided that thedetected voltage does not lie in the valid range if the detected voltagecorresponds to a complete discharge of the capacitance, the time periodafter which the voltage is detected, which differs from thepredetermined time period, being reduced for each repetition.
 4. Amethod according to claim 2, wherein in step a3) it is decided that thedetected voltage does not lie in the valid range if the detected voltagecorresponds to a small discharge of the capacitance, the time periodafter which the voltage is detected, which differs from thepredetermined time period, being increased for each repetition.
 5. Amethod according to claim 2, wherein a photodiode is used as thephotosensitive device, the capacitance being the blocking layercapacitance of the photodiode.
 6. A method according to claim 1, whereinin step b) the valid signal is stored in each case in an intermediatememory together with the time period used for detecting said signal.